In the mixing of dry concrete mixes, and in the mixing of wet concrete, it has been difficult to achieve entirely satisfactory results with conventional methods and equipment. Conventional concrete mixing equipment uses a high energy input compared with the results achieved. Much of the wasted energy is the result of the high friction encountered between the housing and blades, and additional energy is wasted because the mixer operation involves repeated lifting and dropping of the materials being mixed. The equipment is subjected to a high rate of wear, and maintenance costs are usually high. Frequently, the materials are not thoroughly mixed so that the resulting concrete is of non-uniform composition and strength. Concrete mixing is usually a batch operation requiring a substantial amount of time, so that a number of separate mixers must be used if a steady flow of concrete mix or concrete is required. Attempts to mix concrete on a continuous basis have not been successful. Typical concrete mixing methods and apparatus are disclosed in U.S. Pat. Nos. 1,332,938, 2,264,170, 2,610,373, 3,107,901, and 3,306,589.
U.S. Pat. No. 3,942,772 discloses a concrete mixing apparatus and method wherein aggregate and cement are directed to free fall together onto a rotating spiked roller disposed across a traveling air-supported belt. Initial mixing occurs during the free fall of the materials, and mixing is completed at the initial spiked roller and at additional spiked rollers located along the belt. This method and apparatus works reasonably well, but certain difficulties have been encountered during operation. The biggest problem has been the buildup of cement and cement-aggregate mixtures on the interior of the housing for the equipment adjacent the spiked roller onto which the raw materials are fed, caused by the raw materials being thrown against the walls of the housing by rotation of the spiked roller. It is next to impossible to exclude all dampness from the apparatus resulting from atmospheric moisture, which causes the materials thrown against the walls to stick to the walls. Another problem has been the partial separation of the raw materials upon impact of the mixed materials falling onto the spiked roller and belt. The cement tends to separate and become concentrated along the sides of the belt, resulting in a higher concentration of aggregate at the center portions of the belt. This aggrevates the buildup and plugging problems, as the cement, being hydroscopic, becomes sticky and is difficult to mix completely back into the mixture. Mixing would be more satisfactory if the cement-aggregate proportions were constant across the full width of the belt.